Mark,

Whether the nose rises or falls depends on the aeroplane. My SE will dive when I put in down flapperon, and climb when the ailerons are raised (spoilerons or airbrakes). Another model I have (a pattern plane with a very long tail) will climb strongly when the flaps are lowered.

My SE needs about 10% flap to elevator mix, with the elevator going in the same direction as the flapperons to maintain trim. The pattern plane needs something like 40% down elevator to fly level with the flaps down 40°.

I guess it depends whether the wing moment due to the increased lift (forcing the nose down) is greater than the moment between the wing and tail caused by the inceased incidence due to the deployment of the flaps. The wing moment I refer to is the tendency of all lifting surfaces to pitch downward when at a positive angle of attack. The more lift, the more pitching moment, whether the lift increased due to angle of attack or airspeed. Flaps increase the angle of attack since the chord line angles down towards the rear as the flaps deploy.

The SE has a low aspect ratio, so the trailing edge of the wing is relatively a long way behind the CG, and at a large fraction of the distance from the CG to the tail. The pattern plane, with the long tail, and a higher aspect ratio, has the flaps much closer to the CG relative to the tail. Intuitively, you could say the SE flapperons are far enough back to act more like elevons. I suspect you could remove the horizontal tail, reflex the ailerons and the SE would still fly quite nicely.

For those who want to try flapperons on the SE: I find that spoilerons work better for landing. The problem landing the SE is that flies so slowly that it will float from one end of the runway to the other. Raising the ailerons about 10° increases drag, reduces lift and just about kills the floating effect. Landing speed is slightly higher, but it's so slow anyway that landing speed is not an issue.

Spoileron also reduce tip-stalling effects, not that this is really a problem with the SE. Flapperons will decrease washout, so it will be more prone to tip stalling: using aileron in response to a dropping wing will lower the aileron on the wing that is stalling, increasing its effective angle of attack and possibly inducing an even deeper stall.